1. Field of the Invention
This invention relates to a wave energy converter suitable for absorbing ocean wave energy. More specifically, the invention relates to a wave energy converter having a group of at least two floats connected by elongated rigid links disposed at a right angle to the direction of incidence of the waves, which floats are free from the coastal ground and sea bed and are allowed to heave, to sway, and to roll, so that a dynamic system is formed to absorb energy from relative movements between the floats and the links and between the adjacent links, while the natural frequency of this system is approximately tuned to the frequency of the incident wave, whereby the energy absorbing efficiency is remarkably increased and the frequency band having a high energy absorbing efficiency is widened.
2. Description of the Prior Art
Ocean energy is one of the most important natural energy sources and is available in various forms such as tide, ocean currents, streams in the ocean, temperature differences, and waves. Among the different forms of ocean energy, wave energy exists abundantly, but its exploitation for practical use has been limited to only a few examples such as buoy-type electric generating units of several ten watts to energize the lamps of nautical markers. However, research on the exploitation of wave energy has been very active during the past years; for instance, Salter ducks invented by Stephen Salter of University of Edinburgh in Scotland, Cockerell's rafts, air-buoy devices of Britain's National Engineering Laboratory, and a large power-generating ship "KAIMEI" of the Japanese Marine Science and Technology Center of Science and Technology Agency. Recently, M. Bessho of the Japanese Defence Academy proposed a special structural system including a float (Bessho system) and experiments on the Bessho system have been undertaken at Mie Dockyard of Nippon Kokan Kabushiki Kaisha.
Most systems of the prior art to extract ocean wave energy use floats which heave and roll in response to the motion of the incident wave, so that the systems extract wave energy by using the movement of the floats relative to fixed ends thereof secured to coastal ground or the sea bed.
The Bessho system of the prior art uses one float which is secured to the coast or sea bed through a dynamic system including a mass mechanism, a restoring mechanism, and a damping mechanism, so that the energy carried by the wave is transmitted to the dynamic system through the float so as to produce specific phase differences between the float movement and the wave, which phase differences facilitate energy extraction and wave suppression. The physical constants of various elements of the dynamic system are such that the reflected waves and the transmitted waves caused by the float are reduced substantially to zero. Thus, the Bessho system is useful for both wave energy extraction and wave suppression.
The Bessho system is characterized by the following points;
(a) A float floating on the sea is secured to the coastal ground or sea bed through a dynamic system including a restoring mechanism, damping mechanism, and a mass mechanism. PA1 (b) Ocean waves cause the float to heave, to sway and to roll, and if various physical constants of the dynamic system are properly selected, the natural frequency of the fluid-float system can be tuned to that of the incident wave. PA1 (c) When the aforesaid tuning is established, the reflected waves and the transmitted waves produced by the float are minimized substantially to zero, so that the entire energy carried by the incident wave can be extracted. PA1 (i) Since the fluid-float system has only one natural frequency, the energy absorbing efficiency can be maximized as long as the one natural frequency of the fluid-float system is tuned to the major frequency of the incident wave, but when the nature of the incident wave is changed due to variations of the meteorological conditions or other causes, the major frequency of the incident wave fluctuates and the energy absorbing efficiency is reduced greatly. This is due to the fact that the energy absorbing efficiency curve of the Bessho system has a comparatively sharp peak at the tuned frequency of the incident wave. Accordingly, it is apparent that the Bessho system necessitates preparation of a plurality of float systems with different natural frequencies so as to facilitate interchange of the float systems when the meterological conditions are changed. PA1 (ii) The wave energy is absorbed by a dynamic system disposed between the float and a fixed end thereof at the coastal ground or the sea bed, and the float tends to be forced to move excessively away from the fixed end thereof when being exposed to excessively high tide, extraordinarily high waves, and typhoons. Thus, the dynamic system may destroy itself when extraordinary forces act thereon, and it is hard to protect the dynamic system against foreseeable severe conditions at reasonable cost. Full protection of the dynamic system results in an unduly high construction cost.
Theoretically, the Bessho system is advantageous in that when the natural frequency of the fluid-float system is tuned to the major frequency of the incident wave, the entire energy carried by the incident wave can be extracted from the motions such as heaving, swaying, and rolling of the float relative to the fixed point to which the float is secured through the aforesaid dynamic system (including the damping mechanism, the restoring mechanism, and the mass mechanism), and that the reflected waves and the transmitted waves from the float can be suppressed substantially to zero. However, in practice, the Bessho system has the following shortcomings: namely,